Intraspecific and intra-individual differences in emitted volatile compounds and in surface and phloem sap-related metabolites do not only affect host plant choice of monophagous aphids but may also guide them to the plant part that provides their ideal niche by maximising their fitness. However, little is known about the variation at these different plant appearance levels. We investigated the preferences of the monophagous aphid species Macrosiphoniella tancetaria and Uroleucon tanaceti for different plant parts (inflorescence stems, young and old leaves) of Tanacetum vulgare plants from two chemotypes, testing their reactions towards volatile, surface and phloem sap-related cues. Furthermore, we studied the variation in leaf glandular trichome density as well as in the composition of volatile, stored and phloem exudate-located specialised (secondary) plant compounds from the different plant parts of these chemotypes. Aphid species showed differences in their preferences. Aphids of M. tanacetaria had to assess the entire plant to choose the stem, whereas U. tanaceti only needed volatile cues to locate the old leaves, which are the plant parts representing their respective niches. Volatiles and stored metabolites varied in their composition and concentration between chemotypes. Stored metabolites additionally differed among plant parts, which was reflected in distinct trichome densities. The composition of phloem exudate-located specialised compounds mostly varied among plant parts. These pronounced differences in plant chemistry on multiple levels provide distinct perception levels for aphids probably driving their niche choice. This study demonstrates the importance to consider these multiple levels to elucidate plant-herbivore interactions with high resolution.

中文翻译：

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挥发性的、储存的和韧皮部渗出液的化合物代表影响蚜虫生态位选择的不同外观水平

释放的挥发性化合物以及表面和韧皮部汁液相关代谢物的种内和个体内差异不仅影响单食性蚜虫的寄主植物选择，而且还可能引导它们通过最大化其适应性来提供其理想生态位的植物部分。然而，人们对这些不同植物外观水平的变化知之甚少。我们研究了单食蚜虫物种 Macrosiphoniella tancetaria 和 Uroleucon tanaceti 对来自两种化学型的 Tanacetum 植物不同植物部分（花序茎、幼叶和老叶）的偏好，测试了它们对挥发性、表面和韧皮部汁液相关线索的反应。此外，我们研究了叶腺毛状体密度以及挥发分、储存和韧皮部渗出液位于这些化学型的不同植物部分的专门（次生）植物化合物。蚜虫物种在偏好上表现出差异。M. tanacetaria 的蚜虫必须评估整个植物才能选择茎，而 U. tanaceti 只需要挥发性线索来定位老叶，这是代表各自生态位的植物部分。挥发物和储存的代谢物在化学型之间的组成和浓度不同。储存的代谢物在植物部位之间也不同，这反映在不同的毛状体密度上。位于韧皮部渗出液的特殊化合物的组成在植物部位之间大多不同。植物化学在多个层面上的这些显着差异为蚜虫提供了不同的感知水平，可能推动了它们的利基选择。